(a) Field of the Invention
The present invention relates to an image display device, display panel, and driving method thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display device.
(b) Description of the Related Art
The organic EL display device, which is a display device for electrically exciting a fluorescent organic compound to emit light, has organic light-emitting cells that are voltage- or current-driven to display an image. These organic light-emitting cells have, as shown in FIG. 1, a structure composed of an anode (indium tin oxide (ITO)) layer, an organic thin film, and a cathode (metal) layer. For a good balance between electrons and holes to enhance luminescent efficiency, the organic thin film has a multi-layer structure that includes an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL). The multi-layer structure of the organic thin film may also include an electron injecting layer (EIL) and a hole injecting layer (HIL).
There are two driving methods for these organic light-emitting cells: one is a passive matrix driving method and the other is an active matrix driving method using thin film transistors (TFTs). In the passive matrix driving method, anode and cathode electrodes are arranged perpendicular to each other to selectively drive the lines. On the other hand, in the active matrix driving method, a thin film transistor and a capacitor are coupled to ITO pixel electrodes so as to sustain a voltage by the capacity of the capacitor. According to the form of the signals applied to the capacitor to sustain the voltage, the active matrix driving method can be divided into a voltage programming method and a current programming method.
The voltage programming method is for displaying an image by applying a data voltage representing gradation to the pixel circuit, but this method has a non-uniformity problem due to a deviation of the threshold voltage of the driving transistor and the electron mobility. The current programming method is for displaying an image by applying a data current representing gradation to the pixel circuit, guaranteeing uniformity, but this method is problematic in securing the time for charging the load of the data lines since only a small amount of current is used in controlling the organic EL element.
A pixel circuit for compensating for the threshold voltage of the driving transistor in the voltage programming method is disclosed in U.S. Pat. No. 6,362,798, for example.
The pixel circuit disclosed in U.S. Pat. No. 6,362,798 includes, as shown in FIG. 2, four transistors M1 to M4, and an organic EL element (OLED). The driving transistor M1 transfers a current corresponding to a voltage between its gate and source to the OLED. A capacitor Cst is connected between the gate and the source of the driving transistor M1. The transistor M2 is diode-connected and has a gate coupled to the gate of the transistor M1. The switching transistor M3 transmits the data voltage applied to the data line Dm to the transistor M2 in response to the select signal provided by a current scan line Sn, and the switching transistor M4 transmits a precharge voltage Vp to the transistor M2 in response to the select signal provided by a previous scan line Sn-1.
In the conventional pixel circuit shown in FIG. 2, the transistor M2 has the same characteristic as that of the transistor M1 such that the deviation of the threshold voltage of the transistor M1 is compensated. Further, the precharge voltage is appropriately established, and the transistor M2 is coupled in the forward direction.
However, the conventional pixel circuit has a problem of current leakage through the transistor M4 because of the precharge voltage Vp when a high level select signal is applied to the previous scan line Sn-1 to turn off the transistor M4. Accordingly, images with desired gray levels are not displayed because of the current leakage, and unnecessary power consumption is generated in the image display device since the current caused by the precharge voltage is consecutively leaked in the pixel circuit while the precharge operation is not being performed.